Engine systems may be configured with boosting devices, such as turbochargers or superchargers, for providing a boosted aircharge and improving peak power outputs. The use of a compressor allows a smaller displacement engine to provide as much power as a larger displacement engine, but with additional fuel economy benefits. However, compressors are prone to surge. For example, when an operator tips-out of an accelerator pedal, an engine intake throttle closes, leading to reduced forward flow through the compressor, and potentially compressor surge. Surge can lead to NVH issues such as undesirable noise from the engine intake system.
One approach for controlling surge induced noise is shown by Ulrey et al. in U.S. Pat. No. 8,161,746. Therein, when compressor surge conditions are approached, one or most boost actuators are adjusted to control air flow through the compressor. Specifically, an engine intake throttle downstream of the compressor is closed, an additional intake throttle upstream of the compressor is closed, and a compressor bypass valve and wastegate are opened to dump excess boost pressure. By closing the compressor inlet throttle, surge induced noise is reduced.
However, the inventors herein have identified potential issues with such an approach. For example, the approach of Ulrey et al. uses throttle control coupled with a bypass valve to increase compressor flow (so as to address surge) without increasing engine flow. However, due to the delay in compressor bypass valve response, it may be difficult to accurately control the bypass and engine flows under all conditions, engine torque transients may be experienced. As another example, the approach of Ulrey et al. requires the inclusion of an additional throttle upstream of the compressor, and coordination of the upstream throttle with operation of the throttle downstream of the compressor. As such, this adds cost and complexity to the system.
In one example, some of the above issues may be at least partly addressed by a method for a boosted engine system including an electric machine (e.g., a motor). The method comprises, responsive to an indication of turbocharger surge, applying negative torque from an electric machine coupled to the engine while providing a greater than demanded airflow to the engine. In an alternate example, the method includes, responsive to an indication of turbocharger surge, providing a greater than demanded throttle flow to the engine while adjusting an alternate actuator to deliver engine demanded torque based on the greater than demanded throttle flow. For example, one or more of cam timing and spark ignition timing may be adjusted based on the greater than demanded throttle flow. In this way, throttle flow (and compressor airflow) can be modified to mitigate surge, while negative torque is used to meet driver demanded wheel torque while addressing NVH issues associated with the surge.
As an example, an engine system may include a turbocharger, with a compressor driven by a turbine, for providing a boosted intake aircharge. In response to a sudden drop in torque demand, such as during a tip-out, a compressor recirculation valve may be opened to dump boost pressure while a wastegate is opened to reduce turbine speed. In addition, an engine airflow may be reduced to meet the lower torque demand. The resulting reduction in throttle flow can bring the compressor ratio (or compressor flow) within or close to a surge limit. In response to surge being approached or exceeded, an engine controller may limit throttle flow reduction. Specifically, airflow may not be reduced to a level commensurate with the reduced torque demand. Rather, the reduction in airflow may be limited so that the airflow is controlled to a level that is greater than what is actually required to meet the torque demand. Fuel injection may be adjusted based on the airflow to maintain engine combustion at stoichiometry (or an alternate desired combustion air-fuel ratio). The excess torque resulting from the excess airflow may then be absorbed by an electric machine coupled to the engine. For example, where the engine is coupled in a hybrid electric vehicle system, a load from an electric motor may be applied on the engine to add negative torque to the driveshaft and meet the driver torque demand. In further examples, a starter-motor (such as a crank integrated starter generator), or an alternator coupled to the engine may be used to apply a load on the engine and absorb the excess torque. In each case, the amount of negative torque applied may be based on the difference between the operator expected torque (or torque demand) and the torque provided with the engine (while operating with elevated airflow). In one approach, the engine controller may use a reference governor to modify the airflow responsive to the indication of surge. For example, the reference governor may be configured so that the engine airflow is initially modified to reduce compressor surge and then over time, the airflow modification is transitioned to meet driver torque demand.
In an alternate example, while the greater than demanded throttle flow is provided, one or more alternate engine torque actuators may be adjusted to reduce the excess torque. For example, cam timing adjustments and/or spark timing adjustments may be concurrently used to compensate for the excess torque that would have been generated by the greater than demanded throttle flow. This may include, for example, retarding spark timing from MBT by an amount based on the greater than demanded airflow to reduce torque. As another example, intake and/or exhaust cam timing may be advanced or retarded (based on engine speed and breathing effects) by an amount based on the greater than demanded airflow to reduce torque.
In this way, by limiting throttle flow reduction response to a drop in torque and boost demand, a sudden change in flow rate and pressure at the compressor is reduced, lowering the likelihood of compressor surge. By moving compressor operation away from a surge limit, surge related issues are alleviated. By compensating for the excess torque by applying an electric machine load on the driveshaft, or adjusting cam and/or spark ignition timing, a “run-on” feel that is created by the raising of throttle flow to levels that are higher than actually desired, can be overcome. As such, this allows surge to be reduced without degrading the operator's drive feel and while meeting the operator torque demand. Overall, engine performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.